Modern metal melting and holding furnaces utilize liquid or gaseous fuels which are delivered, usually in combination with an oxidant, to a plurality of burners which are directly exposed to the material to be processed. Furnaces designed for the processing of metals may operate within a relatively wide range of temperatures related to any of the various metal processing stages and the particular metal or metal alloy to be processed. Furthermore, selective manipulation of various fuels and oxidant compositions, at specified processing temperatures, yields an oxidizing or reducing processing environment. These processing furnaces are often uniquely configured with a variety of burner arrays installed therein, to provide the required heating characteristics. For example, vertical shaft type furnaces for melting metal are well known in the art, as typified by the furnace disclosed in U.S. Pat. No. 4,301,997 assigned to the assignee of this invention. Correct selection of an appropriate fuel/oxidant combination for use at a selected processing temperature and in a desired furnace environment are important factors which materially effect the processing of metals and their alloys.
Most modern premixed gas-fired metal processing furnaces are heated by passing a specified mass flow of a pressurized mixture of fuel and an oxidant through a metered orifice to the combustion chamber of the furnace. Such oxidants include, for example, atmospheric air, gaseous oxygen, or combinations of oxygen containing gases. The mixture is ignited by an appropriate ignition system, causing steady state combustion of that mass flow within the refractory-lined combustion chamber of the furnace. Burner temperature, flame propagation, and flame stability vary with fuel composition, fuel-oxidant ratio, fuel mixture delivery pressure, various orifice dimensions, and the resulting flow characteristics. Accordingly, a measurable change in any of these parameters may cause a related and undesirable variation in temperature, operating environment, or other operating characteristic within the furnace. In particular, an oxidizing, reducing, or neutral (stoichiometric) atmosphere can be approximated by selectively altering one or more of these variables singly or in combination. Heretofore, however, precise achievement of a desired combustion atmosphere has been accomplished on a hit-or-miss basis for two reasons. First, insufficient and uneven premixing of the fuel flow with an oxidant flow may result in an inconsistent or erratic fuel burn due to non-uniform flame propagation following ignition. Second, partial burning of the fuel often occurs as a result of a premix which is overly rich in the oxidant component, in which case the excess oxidant effectively cools the flame. The resulting cooler flame may be inadequate for those process melts which require relatively high flame temperatures to prevent premature solidification and to remelt already solidified material.
It is well known that an increased mass flow of an oxidant, beyond that required for stoichiometric combustion conditions, can enhance the resulting flame temperature, which is necessary for refining those metals and their alloys having elevated melting points. Alternatively, enhanced processing temperatures can enhance production capacity of the shaft furnace. Such processing requires, in combination with a fuel supply, an increase in the mass flow of oxidant supplied to the burner. However, significant additions of oxidant can result in the rapid and undesirable oxidation of the material being processed if such additions are made in an uncontrolled or insufficiently premixed manner.
In addition, it may be desirable to provide increased processing temperatures while maintaining the reducing atmosphere generally required for the processing of readily-oxidized metals such as copper, aluminum, and their alloys. Increased temperatures are also necessary for the efficient processing of the by-product slags of these metals and their alloys. However, accomplishment of such temperatures by oxidant-enrichment is limited to the extent necessary to maintain the reducing atmosphere within the furnace. Thus, an increase in flame temperature is limited by the oxidant component of the premixture mass flow and by the resulting flame shape and chemistry defined by that ignited premixture mass flow. That is, unbalanced gas-mixing results in a non-uniform fuel burn which in turn provides an erratic or uncertain temperature. Such incomplete combustion also results in excess use (waste) of fuel and oxidant. Furthermore, excess oxidant flow may result in undersirable cooling of the burner and/or metal charge. Accordingly, such insufficient control of gas premixing, and mixing within the burner, results in non-optimized burner and flame temperature, thereby providing insufficient heat necessary to meet elevated melt temperature requirements, and compromising metal throughput of the furnace.